This disclosure relates to turbomachinery, and more particularly, the disclosure relates to a blade outer air seal for a gas turbine engine.
Gas turbine engines include a compressor that compresses air, a combustor that ignites the compressed air and a turbine across which the compressed air is expanded. The expansion of the combustion products drives the turbine to rotate, which in turn drives rotation of the compressor.
In order to increase efficiency, a clearance between the tips of the blades in the compressor and the outer diameter of the flowpath is kept sufficiently small. This ensures that a minimum amount of air passes between the tips and the outer diameter. Some engines include a blade outer air seal (BOAS) supported by case structure to further reduce tip clearance.
Typical small engine power turbines use a full, circumferentially unbroken ring BOAS to control the amount of air leakage at the tip of the power turbine blades. As the blades rotate and heat up due to engine operation the tips will rub a very small amount against a honeycomb seal.
The nature of the full ring/rotating blade arrangement makes tip clearance, distance between the top of the blade tip and honeycomb, sensitive to temperature. The clearance between the BOAS and the blade tips is sensitive to the temperature of the gas path at different engine conditions. If the continuous, unitary ring heats up at a faster rate than the rotating blades the tip clearance could increase and cause a drop in efficiency. Conversely, if the blades heat up at a faster rate than the BOAS support structure, the blades can undesirably rub against the BOAS. This arrangement does not allow for much freedom to allow for the tip clearance to be controlled during different power settings in the engine. Thus, it is difficult to accommodate a consistent tip clearance during different power settings in the engine.
A typical full ring BOAS configuration also dictates that the power turbine case must take on a conical shape. With the addition of each stage, the corresponding ring will have a larger diameter than the previous stage, creating a conical shape, driven by the assembly process. This is due to the fact that each ring must be related to the case through lugs, meaning each stage's full ring BOAS must pass under the aft stage's lugs during assembly.